Here is a recipe for using encrypted boot on a Fedora Server system that does not have a monitor or keyboard attached during normal use.

I’ll use Fedora 21 Server, and will have a dedicated encrypted volume group for data but leave the main operating system volume group unencrypted. The encryption key will be stored on a USB memory stick. When it is connected the system will boot normally; otherwise it will wait for a while for it to be connected and finally fall back to emergency mode.